Composite films comprising carbon nanotubes and polymer

ABSTRACT

A process for forming a composite film on a substrate comprises providing a suspension comprising an ionised polymer and functionalised carbon nanotubes in a solvent, at least partially immersing the substrate and a counterelectrode in the suspension, and applying a voltage between the substrate and the counterelectrode so as to form the composite film on the substrate. Electrical charges on the polymer and on the nanotubes have the same sign and the voltage is applied such that the charge on the substrate has the opposite sign to the charge on the polymer and the nanotubes.

TECHNICAL FIELD

The present invention relates to composite films and to processes formaking them.

BACKGROUND OF THE INVENTION

Carbon nanotubes (CNTs) are fullerene-related structures of graphitecylinders and were first synthesized by Iijima (Iijima S, Helicalmicrotubes of graphitic carbon, Nature 1991, 354, 56). Single wallednanotubes (SWNTs) consist of single layers of graphite lattice rolledinto cylinders, whereas multiwalled nanotubes (MWNTs) consist of sets ofconcentric cylindrical shells, each of which resembles a SWNT. Suchunique structure provides the CNTs with exceptional electrical andthermal conductivity, high strength and stiffness and enormous aspectratio. These properties enable the development of electricallyconductive polymeric composites with very low CNT loading and canprovide improved mechanical performance to a polymeric matrix forapplications ranging from electronic to aerospace and automotiveindustry. Potential applications include conductive structure materialsfor aerospace or automotive industry, electromagnetic interface (EMI)shielding materials, dissipative materials and thermal managementmaterials for the microelectronic industry and potential transparentfield emission materials for display and other electronic applications.

According to percolation theory, a three dimensional CNT conductivenetwork in polymer matrix is needed to provide a conductive path. Thepercolation threshold is characterized by a sharp increase inconductivity coinciding with the formation of a three dimensionalconductive network. Thus, a key factor to achieve reasonableconductivity is the proper dispersion of a CNT filler in a polymericmatrix. In past years several techniques have been developed toefficiently disperse CNTs in a polymeric matrix. The commonest method isdirect mixing of CNTs and polymer through melt blending orshear-intensive mechanical stirring (Moisala A, Li Q, Kinloch I A,Windle A H, Thermal and electrical conductivity of single and multiwalled carbon nanotube epoxy composites, Composites science andtechnology, 2006, 66, 1285; Li Z F, Luo G H, Wei F and Huang Y,Microstructure of carbon nanotubes/PET conductive composites fibers andtheir properties, Composites science and technology, 2006, 66, 1002;Sandler J K W, Kirk J E, Kinloch I A, Shaffer, M S P and Windle A H,Ultra low electrical percolation threshold in carbon nanotube epoxycomposites, Polymer, 2003, 44, 5893; Lavin J G and Samuelson H V, Singlewall carbon nanotube polymer composites, U.S. Pat. No. 6,426,134).However this method is generally not very effective at dispersing CNTsin polymers and is limited to thermoplastics or low viscosity polymers.

In another dispersion method, a solvent was employed to lower theviscosity of polymer and facilitate the dispersion of CNTs. With thismethod, CNTs are first exfoliated into an organic solvent underhigh-power ultrasonication. Then the CNT suspension is mixed withpolymer, and the organic solvent is allowed to evaporate (Kim Y J, ShinT S, Choi H D, Kwon J H, Chung Y C and Yoon H G, Electrical conductivityof chemically modified multiwalled carbon nanotube/epoxy composites,Carbon, 2005, 43, 23; Li N, Huang Y, Du F, He X B and Eklund P C,Electromagnetic interference shielding of single walled carbon nanotubeepoxy composites, Nano Lett, 2006, 6, 1141; Connell J W, Smith J G,Harrison J S, Park C, Watson K A, Ounaies Z, Electrically conductive,optically transparent polymer/carbon nanotube composites and process forpreparation thereof, U.S. Pat. 2003/0158323). Compared to the earlierdescribed method, the dispersion of CNTs in the polymer using thismethod is better. However high-power ultrasonication for a long periodof time generally shortens the nanotube length and destroys itsintegrity, which is detrimental to the conductivity of the resultingcomposite. Also, during slow solvent evaporation, nanotubes tend toagglomerate, leading to inhomogeneous distribution in the polymermatrix. Another problem with the solution blending method is the use oftoxic and flammable solvents.

Grunlan et al describe an approach to incorporating CNTs into apolymeric matrix with relatively low percolation threshold based on theuse of latex technology (Grunlan J C, Mehrabi A R, Bannon M V, Bahr J L,Water based single walled nanotube filled polymer composite with anexceptionally low percolation threshold, Adv Mater 2004, 16, 150).Initially, CNTs and polymer particles were uniformly suspended in asolvent. Once most of the solvent had evaporated, the polymer particlesassumed a close-packed configuration with CNTs occupying interstitialspace. Finally, the polymer particles were coalescenced together to forma coherent film locking the CNTs within a segregated three dimensionalnetwork. In this processing method solid polymer particles createdexcluded volume to reduce the free volume available for the CNTs to forma conductive network. As a result, the percolation threshold wassignificantly reduced.

The interfacial interaction between CNTs and a polymeric matrix willaffect the compatibility of CNTs with the matrix, and hence theirdispersion in the matrix. Thus, both modification of the CNTs byfunctionalization of their walls and modification of the polymer matrixhave been employed to promote the dispersion of CNTs.

While the prior dispersion techniques may be generally satisfactory fortheir respective systems, these techniques are quite limited in theirability to fabricate conductive thin films with controlled thickness,especially on irregular-shaped substrates.

Electrophoretic deposition (EPD) is a widely used industrial colloidalprocess to produce thin films on conductive substrates. In EPD, chargedparticles suspended in a liquid medium are attracted and deposited ontoan oppositely charged conductive electrode in a DC electric field (BerraL and Liu M L, Progress in materials science, 2007, 52, 1). EPD hasadvantages of short film formation time, simple apparatus, continuousfabrication, good homogeneity and packing density and suitability formass production as in electric coating industry. Most importantly, itcan be used to fabricate thin film onto variously-shaped surfaces withcontrolled thickness and morphology. Patterned deposition can also beachieved by using masked electrode. EPD has been used to producepre-fabricated CNTs (Boccaccini A R, Cho J, Roether J A, Thomas B J C,Minay E J and Shaffer M S P, Electrophoretic deposition of carbonnanotubes, Carbon, 2006, 44, 3149), and such fabricated CNTs films showgood electron field emission stability under both continuous and pulsedoperations (Gao B, Yue G Z, Qiu Q, Cheng Y, Shimoda H, Fleming L andZhou O, Fabrication and electron field emission properties of carbonnanotube films by electrophoretic deposition, Adv Mater 2001, 13, 1770).

Polyimides (PI) are excellent in heat resistance, chemical resistanceand mechanical properties. They are widely used in aerospace andautomotive industry and also play important role as dielectric layers ina variety of microelectronic devices. In microelectronics industry, PIfilms are commonly produced by film casting of a non-aqueous polyamicacid precursor solution followed by heat curing. The various castingmethods include air spraying, roll coating, brush coating and dipcoating. However, irregular-shaped objects cannot be easily provideduniform coating films by these methods.

To solve this problem, EPD has been employed, and has shown someadditional advantages such as small loss in coating materials anduniform thin film with controlled thickness. A continuous coating of PIonto electrical conductor has been disclosed in U.S. Pat. No. 3,846,269(Marcello N E, Creek T and Phillips D C, Method for continuous coatingof polyimide by electrodeposition). In this method a coated electricalconductor is made by continuously passing a positively chargedelectrical conductor near a negatively charged electrode in a bath of aconducting non-aqueous polyamic acid suspension. A photosensitivepolyimide having oxycarbonyl groups in side chains has been developedand employed to fabricate a patterned PI film through EPD followed byphotolithography (Hiroshi I and Shunichi M, Composition for polyimideelectrodeposition and method of forming patterned polyimide film withthe same, EP 1 123 954).

In the microelectronics industry, the adhesion strength between PI andmetallic substrate is a crucial factor influencing the performance ofelectronic devices. PI is known to adhere poorly to metals, especiallyto copper, and is easily delaminated from a copper substrate. It wasfound that acid groups of polyamic acid can react with copper to producecopper ions. These copper ions can diffuse into the PI layer toaccelerate the oxidation of PI during heat curing at elevatedtemperature (Chamber S A, Loebs V A and Chakravorty K K, Oxidation of Cuin contact with preimidized polyimide J Vac Sci Technol 1990, A8, 875).To prevent the diffusion of copper ion and maintain the adhesionstrength of the PI/copper interface, a barrier film such as Cr, Ni or Tais always inserted between PI and copper (Ghosh M K and Mittal K L,Polyimides: fundamentals and applications, New York: Marcel Dekker,1996). However, this method is not simple or cost effective.Polyvinylimidazole (PVI) and its silane derivatives have been developedto prevent corrosion of PI layer at high temperature (Jang J and EarmmeT, Interfacial study of polyimide/copper system using silane modifiedpolyvinylimidazoles as adhesion promoters, Polymer, 2001, 42, 2871).These materials suppress the corrosion of copper and the diffusion ofcopper ion into PI through complex formation with copper (Xue G, Shi, G,Ding J, Chang W, Chen R, Complex-induced coupling effect-adhesion ofsome polymers to copper metal promoted by benzimidazole, J Adhesion SciTechnol, 1990, 4, 723). On the other hand, silanes are an effectiveadhesion promoter of PI/inorganic interface (Linde H G and Gleason R T,Thermal stability of the silica-aminopropylsilane-polyimide interface, JPolym Sci Chem Ed 1984, 22, 3043). However, the application of PVI andits silane derivatives requires casting these primers onto the coppersubstrate before application of the PI, which makes the processing morecomplicated.

Thus, there is a need for an efficient method to produce CNT-filledcomposite thin films through EPD with tunable thickness and electricalconductivity. There is also a need for a method to increase the adhesionstrength of a polymeric thin film to a metallic substrate throughincorporating CNTs into the film.

OBJECT OF THE INVENTION

It is an object of the present invention to substantially overcome or atleast ameliorate one or more of the above disadvantages. It is a furtherobject to at least partially satisfy at least one of the above needs.

SUMMARY OF THE INVENTION

In a first aspect of the invention there is provided a process forforming a composite film on a substrate comprising:

(i) providing a suspension comprising an ionised polymer andfunctionalised carbon nanotubes in a solvent;

(ii) at least partially immersing the substrate and a counterelectrodein the suspension; and

(iii) applying a voltage between the substrate and the counterelectrodeso as to form the composite film on the substrate;

wherein electrical charges on the polymer and on the nanotubes have thesame sign and the voltage is applied such that the charge on thesubstrate has the opposite sign to the charge on the polymer and thenanotubes.

The following options may be used in conjunction with the first aspect,either individually or in any appropriate combination.

The voltage may be a DC voltage. The voltage may be applied so as tocause the substrate and the counterelectrode to have opposite electricalcharges. The voltage may be applied so as to cause a current to flowbetween the substrate and the counterelectrode. The voltage may beapplied so as to cause a DC current to flow between the substrate andthe counterelectrode.

The functionalised nanotubes and the polymer may both be negativelycharged and the voltage may be applied such that the charge on thesubstrate is positive (i.e. the substrate may function as an anode).Alternatively the nanotubes and the polymer may both be positivelycharged and the voltage may be applied such that the charge on thesubstrate is negative (i.e. the substrate may function as a cathode).

The functionalised nanotubes may comprise multiwalled nanotubes (MWNTs).They may comprise single walled nanotubes (SWNTs). They may comprise amixture of MWNTs and SWNTs. They may be chemically modified nanotubes.They may be nanotubes in which the surface of the nanotubes ischemically modified. They may be MWNTs in which the surface of the MWNTsis chemically modified. They may be surface functionalised carbonnanotubes. They may be electrically charged carbon nanotubes. They maybe oxidised carbon nanotubes. They may be functionalised carbonnanotubes in which the surface of the walls of the nanotubes has beenoxidised. They may be oxidised MWNTs. They may be functionalised MWNTsin which the surface walls of the MWNTs are oxidised.

The functionalised nanotubes may comprise functional groups on thesurface thereof. The functional groups may be electrically charged. Theymay be at least partially ionised. They may be negatively charged. Theymay be anionic. The functionalised nanotubes may have carboxylate groupson the surface thereof. They may have some other anionic group on thesurface thereof or coupled to the surface thereof, e.g. phosphate,sulfate, sulfonate, carbonate, thiocarbonate, dithiocarbonate,thiocarboxylate or dithiocarboxylate. They may have more than one typeof anionic group on the surface or coupled to the surface. When mentionis made of groups on the surface of the nanotubes, the groups may bedirectly attached to the surface, or may be coupled thereto via a linkergroup.

The polymer may comprise a polyamic acid. In this case the process mayadditionally comprise the step of heating the composite film so as toconvert the polyamic acid into a polyimide. The polymer may be a polymerhaving acidic groups and/or anionic groups formed from acidic groups.The groups may be carboxylate, phosphate, sulfate, sulfonate, carbonateor a mixture of any two or more of these.

The substrate may be an electrically conductive substrate. It may be anelectrically non-conductive substrate. It may comprise a non-conductivecoating on a conductive base. It may comprise a conductive coating on anon-conductive base. It may comprise a polymer-coated metal. Thesubstrate may be metallic. It may comprise copper. It may comprisechromium. It may comprise a chromium coated silicon wafer. The substrateshould be capable of becoming electrically charged when connected to aDC voltage source.

The solvent may comprise a polar organic solvent. It may comprise aprotic solvent. It may comprise an aprotic solvent. It may comprise amixture of solvents, at least one of which is a polar organic solvent.

The ratio of nanotubes to polymer in the suspension may be such that thecomposite film is electrically conductive. The ratio of nanotubes topolymer in the suspension may be such that the composite film adheres tothe substrate. The ratio may be between about 0.5% and about 5%, orbetween about 0.5% and about 2%, or about 0.5% to 1.5% on a w/w basis.

Step (i) may comprise combining a suspension of the nanotubes with acolloidal suspension of the polymer. The process may comprise the stepof preparing the colloidal suspension of the ionised polymer bycombining a solution of an acid form of the polymer with a neutralisingagent, said neutralising agent being sufficiently basic to at leastpartially deprotonate the acid form of the polymer to form the ionisedpolymer. The neutralising agent may be in solution in a liquid. Saidliquid may be a poor solvent for the polymer.

Step (i) may comprise preparing a suspension of functionalisednanotubes. It may comprise preparing a suspension of charged nanotubes,e.g. negatively charged nanotubes. This may comprise functionalisationof the nanotubes. It may comprise surface functionalisation of thenanotubes. It may comprise oxidation of carbon nanotubes. It maycomprise surface oxidation of carbon nanotubes. The oxidation may be anacid oxidation. It may use an oxidising acid such as nitric acid. It mayuse a peroxide. It may use a peroxyacid. It may use oxygen.

The voltage may be applied for a sufficient time to form the compositefilm having a predetermined thickness. The process may comprise the stepof selecting a time for application of the voltage so as to obtain adesired thickness of the composite film.

The ratio of carbon nanotubes to polymer may be determined so as toachieve a predetermined electrical conductivity of the film. The processmay comprise the step of selecting a ratio of carbon nanotubes topolymer so as to obtain a desired electrical conductivity of the film.

The process may be such that the composite film has the functionalisedcarbon nanotubes substantially homogeneously distributed through thepolymer.

In an embodiment there is provided a process for forming a compositefilm on a substrate comprising:

(i) providing a suspension comprising an ionised polymer andfunctionalised carbon nanotubes in a solvent;

(ii) at least partially immersing the substrate and a counterelectrodein the suspension; and

(iii) applying a DC voltage between the substrate and thecounterelectrode so as to form the composite film on the substrate;

wherein electrical charges on the polymer and on the nanotubes have thesame sign and the voltage is applied such that the charge on thesubstrate has the opposite sign to the charge on the polymer and thenanotubes.

In another embodiment there is provided a process for forming acomposite film on a substrate comprising:

(i) providing a suspension comprising an ionised polymer andfunctionalised carbon nanotubes in a solvent;

(ii) at least partially immersing the substrate and a counterelectrodein the suspension; and

(iii) applying a voltage between the substrate and the counterelectrodeso as to cause a current to flow between the substrate and thecounterelectrode so as to form the composite film on the substrate;

wherein electrical charges on the polymer and on the nanotubes have thesame sign and the voltage is applied such that the charge on thesubstrate has the opposite sign to the charge on the polymer and thenanotubes.

In another embodiment there is provided a process for forming acomposite film on a substrate comprising:

(i) providing a suspension comprising a negatively charged polymer andnegatively charged functionalised carbon nanotubes in a solvent;

(ii) at least partially immersing the substrate and a counterelectrodein the suspension; and

(iii) applying a voltage between the substrate and the counterelectrodesuch that the charge on the substrate is positive so as to form thecomposite film on the substrate.

In another embodiment there is provided a process for forming acomposite film on a substrate comprising:

(i) providing a suspension comprising a negatively charged polymer andnegatively charged functionalised carbon nanotubes in a solvent;

(ii) at least partially immersing the substrate and a counterelectrodein the suspension; and

(iii) applying a DC voltage between the substrate and thecounterelectrode such that the charge on the substrate is positive so asto form the composite film on the substrate.

In another embodiment there is provided a process for forming acomposite film on a substrate comprising:

(i) preparing negatively charged functionalised carbon nanotubes byoxidation of carbon nanotubes;

(ii) preparing a suspension comprising a negatively charged polymer andthe negatively charged functionalised carbon nanotubes in a solvent;

(iii) at least partially immersing the substrate and a counterelectrodein the suspension; and

(iv) applying a voltage between the substrate and the counterelectrodesuch that the charge on the substrate is positive so as to form thecomposite film on the substrate.

In another embodiment there is provided a process for forming acomposite film on a substrate comprising:

(i) preparing negatively charged functionalised carbon nanotubes by acidoxidation of carbon nanotubes;

(ii) preparing a suspension comprising a negatively charged polymer andthe negatively charged carbon nanotubes in a solvent;

(iii) at least partially immersing the substrate and a counterelectrodein the suspension; and

(iv) applying a DC voltage between the substrate and thecounterelectrode so as to cause a DC current to flow between thesubstrate and the counterelectrode such that the charge on the substrateis positive, so as to form the composite film on the substrate.

In another embodiment there is provided a process for forming acomposite film on a substrate comprising:

(i) preparing a suspension comprising an at least partially ionisedpolyamic acid and negatively charged functionalised carbon nanotubes ina solvent;

(ii) at least partially immersing the substrate and a counterelectrodein the suspension;

(iii) applying a DC voltage between the substrate and thecounterelectrode such that the charge on the substrate is positive, soas to form a film on the substrate, said film comprising the polyamicacid having the fuctionalised carbon nanotubes substantiallyhomogeneously distributed therethrough; and

(iv) heating the film for sufficient time at a sufficient temperature toconvert the polyamic acid to a polyimide.

The invention also provides composite film made by the process of thefirst aspect.

In a second aspect of the invention there is provided a composite filmcomprising functionalised carbon nanotubes dispersed in a polymer, saidnanotubes being substantially homogeneously distributed through thepolymer.

In a third aspect of the invention there is provided a composite filmcomprising carbon nanotubes dispersed in a polymer, wherein the carbonnanotubes comprise functional groups on the surface thereof and thepolymer comprises functional groups.

The nanotubes may be substantially homogeneously distributed through thepolymer. The functional groups on the surface of the carbon nanotubesmay be, or may be derived from, anionic groups. The functional groups onthe polymer may be, or may be derived from, anionic groups.

The following options may be used in conjunction with the second or thethird aspect, either individually or in any appropriate combination.

The nanotubes may have oxygen-containing functional groups on thesurface thereof. They may have carboxylate groups, or groups derivedtherefrom, on the surface thereof.

The film may be electrically conductive. It may have an electricalconductivity of at least about 1×10⁻³ Sm⁻¹ and a carbon nanotube contentof about 0.65 wt %, or about 0.8 wt %, or about 1 wt % or about 1.2 wt %relative to polymer.

The carbon nanotubes may comprise MWNTs.

The polymer may be a polyamic acid or a polyimide.

The carbon nanotubes may be covalently bonded to the polymer.

The film may have a thickness of less than about 100 microns, or in mayhave a thickness of less than about 50 microns, or it may have athickness of about 5 to about 50 microns, or it may have a thickness ofabout 5 to abut 100 microns. It may have uniform thickness.

The film may be disposed on an irregular substrate. It may be disposedon a substrate having an irregular shape. It may be disposed on asubstrate having a non-smooth surface, or having a rough surface, orhaving an irregular surface.

The film may have a tunable thickness.

In an embodiment there is provided an electrically conductive compositefilm comprising functionalised MWNTs dispersed in a polyimide, saidMWNTs being substantially homogeneously distributed through the polymer.

In another embodiment there is provided an electrically conductivecomposite film comprising functionalised MWNTs dispersed in a polyimide,said MWNTs being substantially homogeneously distributed through thepolymer, wherein the MWNTs are covalently bonded to the polymer.

The film may be disposed on a substrate, e.g. an electrically conductivesubstrate. The substrate with the composite film disposed thereon may bean electrode. The electrode may be an anode.

The film may be made by the process of the first aspect of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention encompasses carbon nanotube (CNT)/polymercomposite thin films and their fabrication through electrophoreticdeposition (EPD). These processes enhance the adhesive strength ofpolymeric thin films onto metallic substrates, and thereby broaden therange of their potential applications.

In one embodiment, an EPD suspension comprising negatively chargedfunctionalised CNTs and at least partially ionised polyamic acid (PAA)colloids in methanol/NMP is subjected to a DC electrical field, therebycausing both CNTs and PAA colloids to migrate towards the anode. Thedeposition rate may be dependent on the suspension concentration, DCcurrent and conductivity of the electrode. After imidization of PAAthrough heating curing, a CNT/polyimide (PI) composite film is producedwith tunable thickness and conductivity.

The present invention also encompasses enhancing adhesion strength ofpolymer onto a metallic substrate through incorporating CNTs. In oneembodiment, the adhesion strength of PI onto copper substrate isenhanced. While crack or delamination of PI film coated on coppersubstrate is generally observed if CNTs are not present in the film, theCNT/PI composite film attaches to copper substrate quite stably.

The present invention provides a process for forming a composite film ona substrate.

The first step of the process is to provide a suspension comprising anionised polymer and functionalised carbon nanotubes in a solvent. Theionised polymer will have an electrical charge associated with it. Itmay therefore be regarded as an electrically charged polymer. Theionised polymer may be completely ionised. It may be partially ionised.In some embodiments it will be a cationic polymer. In other embodimentsit will be an anionic polymer. It may have cationic groups on thesurface thereof. It may have anionic groups on the surface thereof.Where mention is made of an electrical charge on the polymer and/or onthe nanotubes, this refers to a net electrical charge thereon. Thefunctionalised carbon nanotubes may have anionic functional groups onthe surface thereof or may have cationic functional groups on thesurface thereof. The suspension may be a stable suspension. It may bestable for at least about 1 day, or at least about 2, 3, 4, 5 or 6 days,or at least about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5 weeks at roomtemperature, or may be stable for a period of about 1 day, or about 2,3, 4, 5 or 6 days, or about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5 weeks atroom temperature. In this context, “stable” refers to a suspension thatshows no visible signs of separation over the stated period withoutstirring. The electrical charges on the polymer and the carbon nanotubeshave the same sign (i.e. either both positive or both negative).Suitable positively charged groups on either the carbon nanotubes or onthe polymer, or on both, include trialkylammonium groups (where thealkyl group is commonly C1 to C6, e.g. methyl, ethyl, propyl, isopropyletc., and may be a mixture of alkyl groups). Suitable negatively chargedgroups on either the carbon nanotubes or on the polymer, or on both,include, independently, carboxylate, phosphate, sulfate, sulfonate,carbonate or a mixture thereof. These groups may be attached directly tothe polymer or nanotubes, or may be attached via a linker, for examplean alkyl or an aryl group. Negative charges on either the nanotubes oron the polymer or on both may be generated by at, least partialdeprotonation of the corresponding acid form of the charged group (e.g.of a carboxylic acid, sulfonic acid etc.). This may be achieved byreaction with a suitable base, e.g. a trialkylamine (such astriethylamine). The degree of deprotonation or ionisation (independentlyfor the nanotubes and for the polymer) may be between about 5 and about100%, or about 5 to 90, 5 to 50, 5 to 30, 5 to 20, 10 to 100, 10 to 50,50 to 100, 80 to 100, 90 to 100, 10 to 90, 20 to 80, 15 to 33, 15 to 30,20 to 50 or 50 to 80%, e.g. about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50,55, 60, 65, 70, 75, 80, 85, 90, 95 or 100%. The degree of deprotonationor ionisation may be sufficient that, when exposed in suspension to anelectrode having an opposite charge, the nanotubes and/or polymer candeposit on said electrode as a result of their electrostatic attraction.

The nanotubes may comprise multiwalled nanotubes (MWNTs). They maycomprise single walled nanotubes (SWNTs). They may comprise a mixture ofMWNTs and SWNTs. They may be oxidised carbon nanotubes. The nanotubesmay have carboxylate groups on the surface thereof. They may have someother anionic group on the surface thereof, e.g. phosphate, sulfate,sulfonate, carbonate or a mixture of anionic groups. They may have morethan one of these groups on the surface. The functional groups on thesurface of the nanotubes may be such that they inhibit, restrict orprevent aggregation of the nanotubes in the suspension, or in thecomposite film. The concentration of the functional groups on thesurface of the nanotubes may be such that they inhibit, restrict orprevent aggregation of the nanotubes in the suspension, or in thecomposite film. The nanotubes may have a mean length of between about0.5 and about 5 microns, or about 0.5 to 2, 0.5 to 1, 1 to 5, 2 to 5 or1 to 2 microns, e.g. about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5microns.

The polymer may be a thermoset polymer. It may be a thermoplasticpolymer. It may be an electrically insulating polymer. It may be anelectrically non-conducting polymer. The polymer may be any suitablycharged or ionised polymer. The polymer may be a polyamic acid. In thiscase the process may additionally comprise the step of heating thecomposite film so as to convert the polyamic acid into a polyimide. Thepolymer may be a polymer having acidic groups and/or anionic groupsformed therefrom. The groups may be carboxylate, phosphate, sulfate,sulfonate, carbonate or a mixture of any two or more of these. Examplesof suitable polymers include polyamic acids, polyacrylic orpolymethacrylic acid, acrylic or methacrylic acid copolymers,polystyrene sulfonate or styrene sulfonate copolymers etc.

The solvent for the suspension may be a liquid that is a poor solventfor the polymer, allowing formation of a colloidal suspension of thepolymer. In this context, a colloidal suspension of the polymer is takento be a dispersion of particles of the polymer in the liquid. Theparticles are of colloidal size (commonly under about 2 microns in meandiameter) and are described below. The solvent may be a mixed solvent.The solvent may comprise a polar organic solvent. It may comprise amixture of solvents, at least one of which is a polar organic solvent.It may comprise a solvent for the polymer and a poor solvent ornon-solvent for the polymer. These may be in a suitable ratio so thatthe solvent is a sufficiently poor solvent for the polymer as to allowformation of a colloidal suspension of the polymer. For many polymerse.g. polyamic acids, a suitable good solvent is a dipolar aproticsolvent such as N-methylpyrrolidone (NMP) and a suitable poor solvent ornon-solvent is methanol. The colloidal particles of polymer in thecolloidal suspension may have a mean diameter of about 0.2 to about 2microns, or about 0.5 to 2, 1 to 2, 0.2 to 1, 0.2 to 0.5, 0.5 to 1 or0.6 to 0.8 microns, e.g. about 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9,1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2 microns. They mayhave a zeta potential of about −10 to about −30 mV, or about −10 to −20,−20 to −30, −15 to −25 or −18 to −22 mV, e.g. about −10, −15, −16, −17,−18, −19, −20, −21, −22, −23, −24, −25 or −30 mV.

The ratio of nanotubes to polymer in the suspension may be such that thecomposite film is electrically conductive. It may be sufficient toachieve a desired conductivity of the film. Thus the process maycomprise controlling the ratio in order to achieve a desiredconductivity. The ratio of nanotubes to polymer in the suspension issuch that the composite film adheres to the substrate. The ratio may bebetween about 0.5% and about 5% on a w/w basis, or about 0.5 to 4, 0.5to 3, 0.5 to 2, 0.5 to 1.5, 0.5 to 1, 1 to 2, 1 to 5, 2 to 5, 3 to 5, 1to 3 or 1 to 1.5%, e.g. about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5%.The polymer may be present in the suspension at a percentage (w/w orw/v) of about 1 to about 5%, or about 1 to 4, 1 to 3, 1 to 2, 2 to 5, 3to 5, 4 to 5 or 2 to 4%, e.g. about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or5%. The nanotubes may be present in the suspension at a percentage (w/wor w/v) of about 0.005 and 0.1%, or about 0.01 and 0.1, 0.05 and 0.1,0.005 and 0.05 or 0.005 and 0.001%, e.g. about 0.005, 0.01, 0.05 or0.1%.

The suspension may be produced by combining a suspension of the carbonnanotubes with a colloidal suspension of the polymer. It may alsocomprise subjecting the resulting suspension to high shear. The highshear may comprise high shear mixing. It may comprise sonicating theresulting suspension. The sonicating may be conducted for about 5 toabout 30 minutes, or about 10 to 30, 20 to 30, 5 to 20 or 5 to 10minutes, e.g. about 5, 10, 15, 20, 25 or 30 minutes. It may be conductedfor sufficient time to form a substantially homogeneous suspension. Itmay be conducted for sufficient time that the nanotubes are notsubstantially aggregated, or that they are less aggregated than prior tosaid sonicating. It will be understood that practically it is verydifficult to produce completely unaggregated carbon nanotubes. In thecontext of the present specification, the term “not substantiallyaggregated” nanotubes (and related terms) denotes that it is impracticalto further reduce their degree of aggregation substantially. The termshould not be interpreted to mean that no aggregation of the nanotubesis present. The sonicating may be at a frequency of about 20 to about 50kHz, or about 20 to 40, 30 to 50 or 30 to 40 kHz, e.g. about 20, 25, 30,35, 40, 45 or 50 kHz. It may be at a power of about 50 to about 200 W,or about 50 to 150, 50 to 100, 100 to 200, 150 to 200 or 100 to 150 W,e.g. about 50, 100, 150 or 200 W. It may be at a power and frequencysufficient to form a substantially homogeneous suspension. It may be ata power and frequency sufficient that the nanotubes are notsubstantially aggregated, i.e. that it is impractical to substantiallydeagregare them further. It may be at a power and frequency sufficientthat the CNTs are not significantly damaged.

The functionalised carbon nanotubes may be prepared byfunctionalisation, e.g. surface functionalisation, e.g. oxidation ofnormal (i.e. unfunctionalised) carbon nanotubes. This may be achieved byexposing the normal nanotubes to a mild oxidant, for example nitricacid. The nitric acid may be at a concentration of about 1 to about 5M(e.g. about 1 to 3, 3 to 5, 2 to 4 or 2 to 3M, for example 1, 1.5, 2,2.5, 3, 3.5, 4, 4.5 or 5M). The exposing may be accompanied by mildstifling or shaking, and may be conducted for about 0.5 to about 2 hours(e.g. about 0.5 to 1 or 1 to 2 hours, for example about 30, 40 or 50minutes, or about 1, 1.2, 1.4, 1.6, 1.8 or 2 hours). Other oxidationprocesses may be used for oxidising the carbon nanotubes. For examplepotassium chlorate, hydrogen peroxide, oxygen/UV light, acidicpermanganate, chlorosulfonic acid, peracetic acid, peroxymonosulfuricacid, performic acid, perbenzoic acid, persulfate, perborate or otheroxidants may be used to oxidise the carbon nanotubes. The oxidationshould be sufficient to inhibit aggregation of the nanotubes, orsufficient to prevent substantial aggregation of the nanotubes, orsufficient to restrict aggregation of the nanotubes or sufficient toreverse aggregation of the nanotubes. During the oxidation, thesuspension may be subjected to high shear. The high shear may comprisehigh shear mixing. It may comprise sonicating the suspension. Thesonication may be for about 1 to about 3 hours (e.g. about 1 to 2, 2 to3 or 1.5 to 2.5 hours or more than 3 hours, for example about 1, 1.5, 2,2.5, 3, 3.5, 4, 4.5 or 5 hours). This may serve to at least partiallydeaggregate the carbon nanotubes. The sonicating may be at a frequencyof about 20 to about 50 kHz, or about 20 to 40, 30 to 50 or 30 to 40kHz, e.g. about 20, 25, 30, 35, 40, 45 or 50 kHz. It may be at a powerof about 50 to about 200 W, or about 50 to 150, 50 to 100, 100 to 200,150 to 200 or 100 to 150 W, e.g. about 50, 100, 150 or 200 W. It may beat a power and frequency sufficient to form a substantially homogeneoussuspension. It may be at a power and frequency sufficient that thenanotubes are not substantially aggregated. It may be at a power andfrequency sufficient that the nanotubes are less aggregated than priorto said sonicating. The resulting nanotubes may be isolated bycentrifugation, filtration, microfiltration, ultrafiltration, decantingor a combination of these. They may be washed, commonly with water(preferably high purity water such as distilled water or deionisedwater) and dried. Drying may be accomplished by freeze drying, passing adry gas through or over the nanotubes or by some other suitable methodor combination of methods.

Following functionalisation of the carbon nanotubes, as described above,a suspension of the nanotubes may be prepared. This may comprisestirring the nanotubes in a first solvent for a suitable time to achievedispersion (e.g. for about 8 to about 24 hours, or about 8 to 16, 16 to24 or 12 to 18 hours, suitably 8, 10, 12, 14, 16, 18, 20, 22 or 24hours). The first solvent may be a good solvent for the acid form of thecharged polymer. The nanotubes may be present in the first solvent atabout 0.01 to about 1%, or about 0.01 to 0.1, 0.1 to 1 or 0.05 to 0.5%,e.g. about 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1,0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1% on a w/w or w/v basis. Theresulting suspension may then be diluted in a second solvent. The secondsolvent may be a poor solvent or a non-solvent for the acid form of thepolymer. The ratio of first solvent to second solvent may be betweenabout 10 and about 50%, e.g. about 10 to 30, 30 to 40, to 40 or 25 to35% (w/w or v/v). The concentration of carbon nanotubes in the resultingsuspension may be between about 0.01 and about 0.1% w/w or w/v, or about0.01 to 0.05, 0.05 to 0.1, 0.02 to 0.08 or 0.03 to 0.07, e.g. about0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1%. Followingaddition of the second solvent, the suspension may be sonicated forsufficient time to homogenise the nanotubes in the suspension. Typicallythis a quite short period, e.g. about 1 to about 10 minutes, or about 1to 5, 4 to 10, 2 to 8 or 3 to 7 minutes (for example about 1, 2, 3, 4,5, 6, 7, 8, 9 or 10 minutes). Suitable first solvents and secondsolvents may depend on the nature of the polymer used in forming thecomposite film. For polyamic acids, suitable first solvents includedipolar aprotic solvents such as NMP (N-methylpyrrolidone), DMA(N,N′-dimethylacetamide), DMF (N,N′-dimethylformamide), DMSO (dimethylsulfoxide), TMU (tetramethylurea) and tetrahydrothiophen-1,1-oxide, andsuitable second solvents include short chain alcohols such as methanolor ethanol.

The process may also comprise preparing the colloidal suspension of thepolymer. The process for doing so may depend on the nature of thepolymer. One common method for preparing the colloidal suspension of thepolymer is to prepare a solution of an acid (i.e. non-charged) form ofthe polymer in a good solvent and then at least partially neutralise theacid form to form the charged polymer. The polymer itself may beprovided e.g. from commercial sources or may be generated in situ. Forexample a polyamic acid may be generated in situ by reaction of abisanhydride with a diamine. The neutralisation, as mentioned above, maybe between about 5 and about 100%, and accordingly, this step maycomprise combining the solution of the acid form of the polymer withbetween about 0.05 and about 1 mol equivalents of a base capable ofdeprotonating the acid form. The base may be an amine, e.g. a tertiaryamine, or some other suitable base. It may be a hydroxide such as sodiumor potassium hydroxide. It may be triethylamine, trimethylamine,pyridine, aniline, dimethylaniline, N-dimethylethanol, triethanolamine,N-dimethylbenzylamine, and N-methylmorpholine or some other suitablebase. The base may be combined with the solution in neat form or insolution, either in a good solvent for the acid form of the polymer orin a poor solvent or non-solvent for the acid form. The resultingpolymer preparation may be combined with a poor solvent or non-solventfor the acid form of the polymer in order to form a stable colloidalsuspension of the ionised polymer. Thus in one alternative, a solutionof an acid form of the polymer in a solvent is treated with base so asto form the charged polymer, which is less soluble in the solvent andtherefore generates a colloidal suspension. This may be stabilised byaddition of a solvent that is a non-solvent or poor solvent for thecharged polymer. In another alternative, a solution of an acid form ofthe polymer in a solvent is treated with a base so as to form thecharged polymer which remains in solution. The resulting solution isthen combined with sufficient of a solvent that is a non-solvent or poorsolvent for the charged polymer that a colloidal suspension results. Ineither of the above alternatives, the ratio of good solvent to poorsolvent will depend on the solubility of the charged polymer in the twoas well as on the concentration of polymer in the solution. Clearly itis required that the poor solvent or non-solvent is miscible with thegood solvent in the ratio that is used. The ratio may typically be about1:1 to about 1:5, or about 1:1 to 1:3, 1:3 to 1:1, 1:2.5 to 1.3, 1:2 to1:3.5 or 1:2 to 1:4, e.g. about 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4,1:4.5 or 1:5 on a weight or volume basis.

Once the suspension comprising the ionised polymer and functionalisedcarbon nanotubes in a solvent has been prepared, as described above, thesubstrate and a counterelectrode are at least partially immersed in thesuspension. The substrate may be made of any suitable conductivematerial, or may comprise a conductive material coated on a secondsubstance (either conductive or non-conductive). It will be understoodthat only that portion of the substrate that is immersed in thesuspension may be coated with the composite film. The substrate may beany suitable shape. It may be rough or it may be smooth. It may be flator it may be non-flat. It may have sharp edges or it may have no sharpedges. The substrate may be partially immersed in the suspension,whereby the substrate will be only partly coated with the compositefilm. The substrate may be entirely immersed in the suspension, wherebythe substrate may be entirely coated with the suspension. The substratemay comprise a metal. It may comprise copper. It may comprise chromium.It may comprise a metal coated silicon wafer. It may comprise a chromiumcoated silicon wafer. It may comprise a conductive polymer. It maycomprise some other conductive material. It may comprise a blend ofconductive materials (e.g. an alloy). It may comprise more than one ofthe above. It may comprise a conductive material at least partiallycoated with a non-conductive material. For example it may comprise apolymer coated metal. The counterelectrode may be as described above forthe substrate. The substrate and/or the counterelectrode may be cleanedprior to forming the film. The cleaning may for example comprise acidcleaning with a suitable acid that does not dissolve the relevant item.It may also comprise washing the item after the acid cleaning. A spacermay be present between the substrate and the counterelectrode. Thespacer may be made of an electrically non-conductive material e.g. apolymeric material or a ceramic material. The spacer serves to maintaina suitable spacing between the substrate and the counterelectrode. Thespacing may depend on the conditions used in forming the composite filmon the substrate. It may be between about 0.5 and about 5 cm, or about0.5 to 3, 0.5 to 2, 2 to 5, 3 to 5 or 1 to 3 cm, e.g. about 0.5, 1, 1.5,2, 2.5, 3, 3.5, 4, 4.5 or 5 cm.

A voltage (i.e. a potential difference) is then applied between thesubstrate and the counterelectrode so as to form the composite film onthe substrate. Thus in the present invention the formation of the filmon the substrate may be a single step process. The voltage is appliedsuch that the charge on the substrate has the opposite sign to thecharge on the polymer and the nanotubes. Thus, for example, if thepolymer and the nanotubes are both negatively charged, the voltageshould be applied such that the charge on the substrate is positive(i.e. the substrate may function as an anode). In this case thecounterelectrode will be the cathode. The voltage may be applied byconnecting the counterelectrode and the substrate to a voltage source.The voltage should be such that the sign of the charge on the substratedoes not vary through the coating process (i.e. it is always positive orit is always negative). The voltage may therefore be a DC voltage andthe voltage source a DC voltage source. The magnitude of the voltage maybe constant or it may vary. It may vary regularly or irregularly. It mayvary monotonically or may vary non-monotonically, e.g. sinusoidally, asa square wave, a saw-tooth wave or in some other manner. The voltage (orthe mean voltage) may be between about 20 and about 400 volts, or about20 to 200, 20 to 100, 20 to 50, 50 to 400, 100 to 400, 200 to 400, 50 to200, 50 to 100 or 100 to 200 volts, e.g. about 20, 30, 40, 50, 60, 70,80, 90, 100, 150, 200, 250, 300, 350 or 400 volts. The current flow maybe between about 1 and about 200 mA, or about 1 to 100, 1 to 50, 1 to20, 1 to 10, 10 to 200, 50 to 200, 100 to 200, 10 to 100, 10 to 50, 50to 100 or 50 to 150 mA, e.g. about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15,20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140,150, 160, 170, 180, 190 or 200 mA. Alternatively there may be negligiblecurrent flow or no current flow. In this case the system may be regardedas a capacitative system in which the electrodes become electricallycharged by application of a suitable voltage, but no substantial currentflows. This may be the case in circumstances in which any one or more(optionally all) of the substrate, the counterelectrode and thesuspension are electrically insulating or have low electricalconductivity. During the step of applying the voltage, the suspensionmay be kept at a temperature of between about 10 and about 50° C., orabout 10 to 35, 10 to 20, 15 to 50, 15 to 35, 25 to 50, 35 to 50, 25 to25 or 25 to 35° C., e.g. about 10, 15, 20, 25, 30, 35, 40, 45 or 50° C.The voltage may be applied for a suitable time to achieve the desiredthickness of film. Clearly the longer the time of applying the voltage,the thicker the film will be. Typical times are from about 30 seconds toabout 30 minutes, or about 1 to 30, 5 to 30, 10 to 30, 20 to 30, 0.5 to20, 0.5 to 10, 0.5 to 5, 0.5 to 2, 0.5 to 1, 1 to 20, 1 to 10 or 5 to 10minutes, e.g. about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27,28, 29 or 30 minutes.

Following formation of the film, the film (on the substrate) may beremoved from the suspension. It may be then washed with a suitableliquid which does not dissolve the film to an appreciable extent.Suitable liquids will depend on the nature of the polymer. They includeshort chain alcohols such as methanol and ethanol. The film may then bedried e.g. in air.

In the event that the polymer is a polyamic acid or some other thermallycurable polymer, it may be advantageous to cure the polymer (in the caseof a polyamic acid, to convert the polymer into a polyimide). This maybe accomplished by heating to a suitable temperature for a suitabletime. The time and temperature will depend on the chemical nature of thepolymer, and may depend on the thickness and other dimensions of thefilm. It may be preferable to heat at different temperaturessequentially, e.g. about 100° C. for about 30 minutes, then about 200°C. for about 30 minutes and then about 280° C. for about 60 minutes.Depending on the nature of the polyamic acid, the temperature requiredto convert to a polyimide may be between about 120 and about 250° C. andfurther baking at higher temperature (e.g. about 250 to about 300° C.)may be advantageous in obtaining good physical properties.

The present invention also provides a composite film comprising carbonnanotubes dispersed in a polymer, said nanotubes being substantiallyhomogeneously distributed through the polymer. The nanotubes may be notsubstantially aggregated within the polymer. The composite film may bemade by the process described above.

The nanotubes may be as described above in conjunction with the process.The film may be electrically conductive. It may have an electricalconductivity of at least about 10⁻³ Sm⁻¹, or at least about 5*10⁻³,10⁻², 5*10⁻², 0.1, 0.5, 1, 5, 10, 50, 100, 200, 500 or 1000 Sm⁻¹. It mayhave an electrical conductivity of about 0.001 to 1000, 0.001 to 100,0.001 to 10, 0.001 to 1, 0.001 to 0.01, 0.01 to 1000, 1 to 1000, 100 to1000, 0.1 to 100, 0.1 to 10, 0.1 to 1 or 1 to 100 Sm⁻¹, e.g. about0.001, 0.002, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20,50, 100, 200, 500 or 1000 Sm⁻¹. The electrical conductivity will dependon the concentration of carbon nanotubes in the film. The nanotubes maybe in sufficient concentration in the film that they form a continuousthree dimensional conductive network. The film may have an electricalconductivity of at least about 1×10⁻³ Sm⁻¹ and a carbon nanotube contentof about 0.65 wt %, or about 0.8 wt %, or about 1 wt % or about 1.2 wt %relative to polymer. Alternatively the conductivity may be less than10⁻³ Sm⁻¹, or less than about 10⁻⁴, 10⁻⁵, 10⁻⁶ or 10⁻⁷ Sm⁻¹ for examplethe conductivity may be about 10⁻⁴, 10⁻⁵, 10⁻⁶ or 10⁻⁷ Sm⁻¹, when carbonnanotube concentrations are used that are insufficient form a continuousthree dimensional conductive network or that are less than about 0.65%relative to polymer.

The carbon nanotubes and the polymer may be as described earlier. Thecarbon nanotubes may have carboxyl groups on the surface thereof. Theymay have ester groups, or may have amide groups. The carbon nanotubesmay be functionalised carbon nanotubes. The carbon nanotubes, or atleast some of the carbon nanotubes, may be covalently bonded to thepolymer. They may be covalently bonded through anhydride linkages, orthrough amide linkages or through ester linkages or through some othersuitable type of linkage.

The film may have a thickness of less than about 100 microns, or lessthan about 50 microns, or less than about 40, 30, 20 or 10 microns, orbetween about 5 and about 100 microns, or about 5 to 50, 5 to 40, 5 to30, 5 to 20, 5 to 10, 10 to 50, 20 to 50 or 10 to 30 microns, e.g. about5, 10, 15, 20, 25, 30, 35, 40, 45 or 50 microns. Alternatively the filmmay have a thickness of at least about 50 microns, or at least about 60,70, 80, 90 or 100 microns, or may be about 50 to 200 microns, or about75 to 200, 100 to 200, 150 to 200, 50 to 100, 75 to 150 or 75 to 100microns, e.g. about 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150,160, 170, 180, 190 or 200 microns. The thickness of the film may betailorable by controlling the conditions and time of film formation. Thefilm may have a substantially uniform thickness.

The film may have no filler other than the carbon nanotubes. It may haveno electrically conductive filler other than the carbon nanotubes. Itmay comprise a non-conductive filler (e.g. silica, talc, calciumcarbonate etc.). It may comprise a second electrically conductive filler(e.g. metallic particles or particles of conductive polymer). In theevent that the film comprises a filler other than the carbon nanotubes,these may have charged functional groups on the surface thereof, orfunctional groups derived from said charged functional groups, whereinthe charge on said charged functional groups is the same sign as thecharge on the carbon nanotubes.

The film may be on a metallic, e.g. copper, substrate. It may have noprimer or other layer between the film and the substrate. It may havegood adhesion to the substrate. Thus in an embodiment of the invention acomposite film comprising carbon nanotubes dispersed in a polyimide islocated directly on a metallic, e.g. copper, substrate, said film havinggood adhesion to the substrate. The film may have sufficiently goodadhesion to the substrate that it does not delaminate from the substrateeither during high temperature formation of the polyimide or duringnormal use of the coated substrate.

The film may have improved physical properties relative to a film of thesame polymer having no CNTs. The film may have improved elastic modulusand/or hardness relative to a film of the same polymer having no CNTs.The improvement may be at least about 5%, or at least about 6, 7, 8, 9,10, 15 or 20% or may be about 5 to about 20%, or about 5 to 15, 5 to 10,10 to 20 or 8 to 13%, e.g. about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,16, 17, 18, 19 or 20%. The improvement may be dependent on the loadingof CNT in the polymer

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described,by way of an example only, with reference to the accompanying drawingswherein:

FIG. 1 is a schematic illustration of the fabrication of conductive CNTspolymer composite through electrophoretic deposition;

FIG. 2 is a schematic illustration of the syntheses of polyamic acid andits neutralization;

FIG. 3 shows photographs of electrophoretic deposited pure polyimidefilm (a) and polyimide/MWNT composite film (b) on copper substrate withfeeding content of MWNT of 0.65 wt % of PAA, current of 15 mA anddeposition time of 3 minutes;

FIG. 4 shows TEM micrographs of MWNT/polyimide composite fabricatedthrough EPD processing with feeding content of MWNT of 0.65 wt % of PAA;

FIG. 5 shows modulus and hardness profiles with respect to displacementinto sample surfaces for neat PI thin film and MWNT/PI composites thinfilm with MWNT feeding content of 0.65 wt % of PAA fabricated onto metalcoated silicon wafer through EPD processing; and

FIG. 6 shows the dependence of the thickness of MWNT/PI composite thinfilm against deposition time (DC current: 15 mA, MWNT feeding content: 1wt % of PAA).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The carbon nanotube (CNTs)/polymer composite thin films disclosed hereinmay be fabricated by electrophoretic deposition (EPD) of CNTs and apolymer colloid suspension in a DC electric field. A schematicillustration of formation of the conductive CNT/polymer composite thinfilm is shown in FIG. 1.

An EPD suspension according to the present invention may comprisenegatively charged CNTs having oxycarbonyl group on their surface. Here,the term “oxycarbonyl group” refers to a group which provides free COO⁻groups in the EPD suspension. A preferred oxycarbonyl group is thecarboxyl group (COOH). The CNTs may be SWNT or MWNT. MWNT are preferredfor fabricating electrically conductive composite thin films.

Carboxylated MWNT may be prepared from commercially available MWNTthrough acid-oxidation. This may be achieved by sonicating the MWNTlightly in the presence of nitric acid or mixed sulfuric acid and nitricacid, followed by thorough washing with water. The degree of the acidtreatment is dependent on treating time, treating temperature and acidconcentration. To maintain the integrity of the graphite structure ofMWNT, use of dilute nitric acid and sonicating for short time arepreferred. The acid-treated MWNT may be thoroughly dried, e.g. by freezedrying followed by further drying at high temperature under high vacuum.Alternatively, the carboxylated CNTs may be prepared through a radicalreaction as reported previously (Peng H Q, Alemany L. B., Margrave J. L.and Khabashesku V. N., Sidewall carboxylic acid functionalization ofsingle walled carbon nanotubes, J Am Chem Soc, 2003, 125, 15174).

The MWNT suspension for EPD may be prepared by suspending acid-treatedMWNT in a polar organic solvent. The polar organic solvent may be thesame as that used in preparation of polyamic acid (PAA) suspension. TheMWNT suspension may be prepared under high power sonication for about 1to 6 minutes.

The EPD suspension according to a preferred embodiment of the presentinvention comprises a negatively charged polyamic acid (PAA) colloidwhich is obtained by neutralisation of PAA. PAA may be obtained fromcommercially available sources or by laboratory synthesis. For example,Pyre-ML® products, which are part of a family of materials based onaromatic polyimides, can be purchased from Industrial Summit TechnologyCo. These products include Pyre-ML® Wire Enamels, Liquid ‘H’ Enamel,Insulating Varnish, and Thinner for different application.

To synthesize PAA, substantially equal amounts of an aromatictetracarboxylic dianhydride and an aromatic diamine (on a molar basis)are subjected to polycondensation in an organic polar solvent at roomtemperature. Molecular weight control may optionally be performed byadjusting the stoichiometry of the acid dianhydride and diamine oradding a terminator such as maleic anhydride. Thus the molar ratio maybe about 0.8 to about 1.2, or about 0.9 to about 1.1, or about 0.95 toabout 1.05, e.g. about 0.8, 0.85, 0.9, 0.95, 0.96, 0.97, 0.98, 0.99, 1,1.01, 1.02, 1.03, 1.04, 1.05, 1.1, 1.15 or 1.2.

While there are no particular limitations on the aromatictetracarboxylic dianhydride that may be used, the following compoundsare provided as typical examples: pyromellitic dianhydride,3,4,3′,4′-biphenyltetracarboxylic dianhydride,3,4,3′,4′-benzophenonetetra-carboxylic dianhydride, and4,4′-(hexafluoroiso-propylidene)diphthalic anhydride. While there are noparticular limitations on the aromatic diamines that may be used, thefollowing are provided as typical examples: 1,2-, 1,3- and1,4-phenylenediamine, 2,4-diamino-toluene, 2,5-diaminotoluene,4,4′-oxydianiline, and2,2-bis[4-(4-aminophenoxy)phenyl]hexa-fluoropropane. Typical organicpolar solvents include N-methylpyrrolidone, N,N′-dimethylacetamide,N,N′-dimethylformamide, dimethyl sulfoxide, tetramethylurea,tetrahydrothiophen-1,1-oxide. Preferably, less toxicN-methyl-pyrrolidone or N,N′-dimethylformamide is used.

PAA colloids are prepared by dropping the above polyamic acid solutioninto a poor solvent comprising a neutralizing agent to ionize the PAA.Alternatively the polyamic acid may be neutralised by combining thesolution with a neutralizing agent and the resulting mixture added tothe poor solvent. Water, aliphatic alcohols, benzyl alcohol andsubstituted benzyl alcohol are typical examples of poor solvents whichare particularly suitable for use in the formation of stable emulsion.Methanol is suitable. Typically, the weight ratio of poor solvent topolar solvent ranges from 2 to 3.5, preferably, from 2.5 to 3. Exemplaryneutralizing agents include N-dimethyl ethanol, triethylamine,triethanolamine, N-dimethyl-benzylamine, and N-methylmorpholine.Triethylamine is a preferred neutralizing agent. Typically, ionizedparts of polyamic acid range from 10 to 50 wt %, preferably, from 15 to33 wt %. FIG. 2 shows the synthesis of PAA and its neutralization by abasic compound leading to ionized PAA.

The EPD suspension according to the present invention is prepared bymixing acid-treated MWNT suspension and PAA colloids under water bathsonicating for 5 to 30 minutes. 20 minutes is a preferred time. Thesolid content of PAA in the electrodeposition bath, i.e. the EPDsuspension, may be adjusted from about 1 to 5 wt %, preferably fromabout 2 to 3.5 wt %; the content of MWNT may be adjusted from about 0.5to 2.0 wt % of PAA, preferably about 0.6 to 1.2 wt %. The content ofMWNT may be lower than 0.5 wt % if a film of lower conductivity isrequired. The suspension thus prepared has good storage stability. In aclosed container, it may be stored stably at room temperature for up totwo weeks.

General processes of electroplating polyimides which are similar toheretofore known EPD processes may be applied without substantialmodification. In an example, a piece of metal coated silica wafer as acathode and a workpiece (metal coated silica wafer or copper plate) asan anode are immersed in the above suspension bath at a temperaturerange of about 15-35° C. and a current range of about 1-200 mA,preferably about 2-20 mA, or a voltage range of about 20-400 volts,preferably about 20-200 volts, for a period of about 30 seconds to 20minutes, preferably about 1-10 minutes. The deposited film is thenwashed with methanol, and if necessary imidization was accomplished inan air oven at about 100° C. for about 30 minutes, about 200° C. forabout 30 minutes, and about 280° C. for about 60 minutes. The depositionmay be carried out without stirring or slight stirring. Alternatively,the deposition may be conducted using suspension flow motivated byelectrical pump. The latter alternative may be preferred in large scalesystems which employ a large amount of suspension. Thus the EPDsuspension may be agitated or may be not agitated.

Thus a representative procedure for producing a composite film accordingto the present invention comprises the following steps:

Preparation of charged carbon nanotubes: stirring a mixture ofmultiwalled carbon nanotubes in dilute aqueous nitric acid (about 1 toabout 5M) for about 0.5 to about 2 hours, followed by sonication forabout 1 to about 3 hours, filtering, washing and drying the solids;

Preparation of charged polymer colloids: reacting approximatelyequimolar amounts of a diamine and a bis-anhydride in a solvent forabout 0.5 to about 2 days at room temperature, at least partialneutralisation of the resulting polyamic acid with a triorganoamine(e.g. triethylamine), and addition to the resulting solution ofsufficient quantity of a poor solvent or non-solvent for the resultinganionic polyamic acid to cause formation of a colloidal suspension ofthe anionic polyamic acid;

Preparation of suspension comprising negatively charged polymer andnegatively charged carbon nanotubes: optionally adding furthernon-solvent or poor solvent to the colloidal suspension of polymer,suspending the carbon nanotubes in a solvent, commonly a mixed solventhaving approximately the same composition as that of the colloidalsuspension of the polymer, and combining the suspension of polymer andthe suspension of carbon nanotubes in a suitable ratio to achieve thedesired ratio of carbon nanotubes to polymer (commonly the ratio ofcarbon nanotubes to polymer is about 0.2 to about 2 wt %);

Film formation: sonicating the suspension comprising negatively chargedpolymer and negatively charged carbon nanotubes for about 15 to about 30minutes, immersing a substrate anode and a cathode into the suspensionand applying a DC voltage sufficient to achieve a current of about 10 toabout 20 mA across the electrodes for about 1 to about 5 minutes inorder to form a composite film on the anode.

The EPD procedure is illustrated in FIG. 1. As shown in FIG. 1,initially the MWNT and PAA colloid are uniformly suspended inmethanol/NMP. Under the influence of an electric field it is envisagedthat negatively charged PAA colloid and MWNT migrate to opposite chargedanode (the workpiece) and closely pack on the surface of the workpiece.It is thought that because of the excluded volume created by PAAparticles, the MWNT are pushed into the interstitial space between PAAparticles, which dramatically reduces the space available for MWNT toform conductive networks, resulting in a reduced percolation threshold.After heat curing at high temperature, a coherent MWNT/PI composite filmis prepared.

FIG. 3 shows optical photographs of fabricated MWNT/PI composite thinfilm and pure PI thin film on copper. While a delaminated or evencracked pure PI thin film is observed as shown in FIG. 3 a, a smoothMWNT/PI composite thin film is observed in FIG. 3 b, indicating thatincorporation of MWNT into PI by means of EPD enhances the thermalstability and adhesion strength of PI film onto the metallic substrate.The poor adhesion of PI to copper is thought to be due to the increasedcorrosion of PI in the presence of Cu²⁺ at high curing temperature. Theimproved adhesion of the films of the present invention may relate tothe formation of surface hydroxides on the electrode (copper) under theinfluence of the electric field. These may then hydrogen bond to theacid treated MWNT. It is also possible that residual carbonyl groups orhydroxyl groups on the MWNT surface prevent the diffusion of Cu²⁺ intoPI layer through their strong interaction with Cu²⁺.

During EPD processing, an anode reaction is thought to lead to theregeneration of the COOH from COO⁻ on the surface of MWNT. Some of theresulting COOH groups from the MWNT may be involved in imidization withPI. As a result, the interfacial interaction between MWNT and PI isthought to be enhanced by the formation of amide groups, which furtherfacilitates MWNT dispersion in PI. The microstructure of a preparedMWNT/PI composite may be observed by using TEM. Examples of such TEMmicrographs are shown in FIG. 4. The TEM results suggest that aneffective MWNT dispersion of conductive network is developed by means ofEPD processing.

The mechanical properties of neat PI and MWNT/PI composite thin filmsfabricated through EPD processing were characterized by nanoindentationtesting. FIG. 5 shows the modulus (E) and hardness (II) profiles withrespect to the indentation depth, respectively, for neat PI and MWNT/PIcomposite thin film with MWNT loading of 0.65 wt % relative to PAA. Itcan be seen that both the elastic modulus and the hardness of MWNT/PIcomposite thin film are improved by about 10% compared with a neat PIthin film, suggesting that incorporation of MWNT has enhanced themechanical property of PI. This may also relate to the thermal stabilityof MWNT/PI composite thin film.

The thickness of fabricated film may be varied by changing one or moreof the suspension concentration, DC current, conductivity of theelectrode and deposition time. FIG. 6 shows the thickness of MWNT/PIcomposite thin film fabricated onto Cr coated silicon wafer through EPDprocessing with DC current of 15 mA and MWNT feeding content of 1 wt %of PAA with various deposition times. It can be seen that film thicknessincreases from about 16 μm to about 55 μm with deposition time increasesfrom about 1 is minute to about 4 minutes, suggesting that the EPDprocessing is capable of producing thin film with tunable thickness. Thethickness is also influenced by the conductivity of substrate. Forexample, while the thickness is about 55 μm for film deposited onto Crcoated silicon wafer (deposition time 4 minutes), it is about 73 μm forfilm deposited onto copper with MWNT feeding content of 1 wt % of PAA.

The neat PI film is electrically insulating. In contrast, the MWNT/PIfilm fabricated through EPD processing is electrically conductive whenthe MWNT feeding content is 0.65 wt % of PAA or above. Table 1 shows theelectrical conductivity of MWNT/PI composite thin films fabricated ontoCr coated silicon wafer through EPD processing with DC current of 15 mAfor 4 minutes but with different MWNT content. It can be seen that theconductivity increases from 1×10⁻³ Sm⁻¹ to 1.6×10² Sm⁻¹ with the MWNTcontent increases from 0.65 to 1.2 wt % of PAA, suggesting the EPDprocessing can produce thin film with tunable conductivity. This isbecause at higher MWNT loading, more MWNT migrate to anodic electrodeand deposit with PI colloids to form MWNT/PI composite thin film withhigher MWNT content.

TABLE 1 Conductivity of MWNT/PI thin film fabricated onto Cr-coatedsilicon wafer with different MWNT feeding content. MWNT content (wt %)0.65 0.8 1.0 1.2 Conductivity (Sm⁻¹) 1 × 10⁻³ 2.1 × 10⁻² 5 × 10⁻¹ 1.6 ×10²

This invention is further described by the following examples forembodiment but not limiting its scope.

Example 1

600 mg of pristine MWNT was mixed with 240 ml nitric acid (2.6 M). Themixture was stirred for 1 hr using mechanical stirrer at rate of 400rpm. After that, the mixture was stirred for 2 more hrs under sonicatingin a water bath ultrasonicator. The mixture was then vacuum-filteredthrough a 0.22 μm polycarbonate membrane, and washed with distilledwater until the pH value of the filtrate was ca. 7. Finally theacid-treated MWNT was freeze-dried and then further dried at 45° C. for48 hrs under high vacuum.

Example 2

4.2 mg acid-treated MWNT was suspended in 5.0 g NMP by stirringovernight. Then 15.0 g CH₃OH was added and the resulting mixture wassonicated for 5 minutes using high power sonicator to prepare a MWNTsuspension for EPD processing. MWNT suspensions with different MWNTcontents were prepared by changing the MWNT weight in the aboveprocedure.

Example 3

In a 250 ml three-neck round-bottom-flask, 10.0 g 4,4′-oxydianiline(ODA, 0.05 mole) and 140 ml N-methylpyrrolidone (NMP) were mixed. Afterthe ODA was completely dissolved in the NMP, 10.9 g pyromelliticdianhydride (PMDA, 0.05 mole) was added and kept stirring for 24 hours.Finally, the resulting polyamic acid (PAA), PMDA-ODA, was store in ablue cap reagent bottle in a freezer below −10° C.

Example 4

10 g prepared PMDA-ODA NMP solution (12.65 wt %) according to Example 3was diluted with 6.5 ml NMP and neutralized by adding 169 μLtriethylamine. The ionized carboxylic group was about 20%. Then, stablecolloidal EPD suspension was prepared by dropping 58.8 ml methanol(weight ratio of methanol and NMP about 3) into the above PMDA-ODA NMPsolution. The final content of PAA was 2 wt %. The effective diameterand the average zeta potential of the prepared PAA particles were about0.7 μm and −20.5 mV, respectively, measured by Zetaplus Particle Sizingequipment.

Example 5

The PAA colloid prepared according to Example 4 was diluted to 1 wt % bymixing with equal volume of CH₃OH/NMP with weight ratio of 3 and putinto a beaker and its temperature was controlled by further putting thebeaker into a water bath. A Cr-coating silica wafer used as a cathodeand a workpiece (metal coated silica wafer or copper plate) as an anodewere immersed into the EPD suspension and separated by a spacer withlength of 1.5 cm (i.e. of suitable dimensions to provide a spacingbetween the coated silica wafer and the workpiece of about 1.5 cm).Then, EPD processing was carried out under a constant electric currentgenerated by a DC source supplied by of LAMBDA Invensys Genesys™. EPDprocessing was conducted at different DC current and deposition time.

The EPD suspension for neat PI film had the following characteristics:polyamic acid content 1%; ionized parts 15-33%; weight ratio of methanolto NMP 3.0; temperature, 22° C.

Example 6

The EPD suspension prepared according to Example 4 was mixed with equalvolume of MWNT suspension prepared according to Example 2 under waterbath sonication for 20 minutes. The obtained mixed suspension was thentransferred to a beaker and its temperature was controlled by furtherputting the beaker into a water bath. A Cr-coating silica wafer used asa cathode and a workpiece (metal coating silica wafer or copper plate)as an anode were immersed into EPD suspension and separated by a spacerwith length of 1.5 cm. Then, EPD processing was carried out under aconstant electric current generated by a DC source supplied by LAMBDAInvensys Genesys™. EPD processing was conducted at varied DC current,deposition time and MWNT content.

The EPD suspension for MWNT/PI film had the following characteristics:polyamic acid content 1 wt %; ionized parts 15-33%; weight ratio ofmethanol to NMP 3.0; MWNT content 0.5 to 1.5 wt % relative to polymer;temperature, 22° C.

1. A process for forming a composite film on a substrate comprising: (i)providing a suspension comprising an ionised polymer and functionalisedcarbon nanotubes in a solvent; (ii) at least partially immersing thesubstrate and a counterelectrode in the suspension; and (iii) applying avoltage between the substrate and the counterelectrode so as to form thecomposite film on the substrate; wherein the electrical charges on thepolymer and on the nanotubes have the same sign and the voltage isapplied such that the charge on the substrate has the opposite sign tothe charge on the polymer and the nanotubes.
 2. The process of claim 1wherein the polymer and the nanotubes are both negatively charged andthe voltage is applied such that the charge on the substrate ispositive.
 3. The process of claim 1 or claim 2 wherein the nanotubescomprise multiwalled nanotubes (MWNTs).
 4. The process of any one ofclaims 1 to 3 wherein the nanotubes have carboxylate groups on thesurface thereof.
 5. The process of any one of claims 1 to 4 wherein thepolymer is a polyamic acid.
 6. The process of claim 5 additionallycomprising the step of heating the composite film so as to convert thepolyamic acid into a polyimide.
 7. The process of any one of claims 1 to6 wherein the substrate comprises copper.
 8. The process of any one ofclaims 1 to 7 wherein the solvent comprises a polar organic solvent. 9.The process of any one of claims 1 to 8 wherein the ratio of nanotubesto polymer in the suspension is such that the composite film iselectrically conductive.
 10. The process of any one of claims 1 to 9wherein the ratio of nanotubes to polymer in the suspension is such thatthe composite film adheres to the substrate.
 11. The process of any oneof claims 1 to 10 wherein step (i) comprises combining a suspension ofthe nanotubes with a colloidal suspension of the polymer.
 12. Theprocess of claim 11 comprising the step of preparing the colloidalsuspension of the ionised polymer by adding a solution of an acid formof the polymer to a mixture of a neutralising agent and a liquid, saidliquid being a poor solvent for the polymer and said neutralising agentbeing sufficiently basic to at least partially deprotonate the acid formof the polymer to form the ionised polymer.
 13. The process of any oneof claims 1 to 12 wherein the voltage is applied for a sufficient timeto form the composite film having a predetermined thickness.
 14. Theprocess of any one of claims 1 to 13 wherein the ratio of carbonnanotubes to polymer is determined so as to achieve a predeterminedelectrical conductivity.
 15. A composite film made by the process of anyone of claims 1 to
 14. 16. A composite film comprising functionalisedcarbon nanotubes dispersed in a polymer, said nanotubes beingsubstantially homogeneously distributed through the polymer.
 17. Thecomposite film of claim 16, said nanotubes having carboxylate groups orgroups derived therefrom on the surface thereof.
 18. The composite filmof claim 16 or claim 17, said film being electrically conductive. 19.The composite film of any one of claims 16 to 18 wherein the carbonnanotubes comprise MWNTs.
 20. The composite film of any one of claims 16to 19 wherein the polymer is a polyamic acid or a polyimide.
 21. Thecomposite film of any one of claims 16 to 20 wherein the carbonnanotubes are covalently bonded to the polymer.
 22. The composite filmof any one of claims 16 to 21 having a thickness of less than about 100microns.
 23. A composite film comprising carbon nanotubes dispersed in apolymer, wherein the carbon nanotubes comprise functional groups on thesurface thereof and the polymer comprises functional groups, wherein thefunctional groups on the surface of the carbon nanotubes are, or arederived from, anionic groups, and the functional groups on the polymerare, or are derived from, anionic groups.